1. Field of the Invention
This invention relates to magnetic read heads which utilize Hall effect elements to detect changes in the magnetic field of a magnetic recording medium.
A Hall device is one which develops an electrical voltage along a first axis upon being biased with electric current along a second axis when a magnetic field is passed through a third axis; the first, second and third axes being at right angles to each other. The output signal is the electric voltage along the first axis and is proportional to the magnetic field.
2. Description of the Prior Art
Materials exhibiting the Hall effect have been utilized in magnetic read heads for many years. Generally, thin film semiconductor materials such as InSb or InAs have been used. One typical head design, shown in FIG. 1, utilizes a conventional ring-type recording head 10 having a Hall element 12 located in the magnetic circuit of the head, often in the rear gap of the head. Movement of a recording medium 14 past the head induces a flux in the magnetic circuit which then passes through the Hall element 12, thereby creating a voltage output. It is the induced flux in the core which is sensed by the Hall element, rather than the actual flux of the recording medium. Heads of this type are disclosed in U.S. Pat. Nos. 3,114,009 to Camras et al. and 3,943,570 to Yamamoto et al. A Hall modulator incorporating a similar construction is disclosed in U.S. Pat. No. 3,221,273 to Livingston. However, the fabrication of a head which incorporates a magnetic core as well as a Hall element is cumbersome and such a device does not lend itself to miniaturization. This concept is therefore unsuitable for high-track-density, multitrack heads, such as are required for digital audio and video tape recorders. Magnetoresistive read elements can be used for this latter application. However, because their dimensions are comparable to the size of the magnetic domains, they are prone to noise and instabilities caused by the Barkhausen effect (sudden rearranging of the domains); moreover, their resolution is inferior to that of the Hall heads which are the subject of this invention.
In order to overcome the problems associated with utilizing magnetic core materials in Hall effect read heads, heads have been developed in which the Hall element is subjected to the magnetic field of the recorded medium itself and no magnetic core elements are utilized. Devices of this type are disclosed in U.S. Pat. Nos. 3,668,670 to Andersen and 3,622,898 to Salmon. In the heads disclosed therein, the thin film Hall element is located so that its major plane is parallel to the surface of the recording medium. The head operates by detecting the portion of the magnetic field of the medium which is perpendicular to the surface of the medium. As shown in FIG. 2, current is made to flow through the Hall element 16 in a direction shown by an arrow 18 such that a magnetic field passing up from the recording medium 20 perpendicular to the plane of the Hall element 16 will produce an output voltage Vo. The position of the Hall element 16 when sensing a flux transition between two regions of mutually opposed magnetization 20a and 20b is shown in FIG. 3. The Hall element 16 senses the component of the magnetic field perpendicular to the surface of the recording medium 20, and the physical orientation of the Hall element is such that the resolution of the head (ability to read very densely recorded information) is limited.
Hall effect read heads have also been proposed which incorporate dual Hall elements. Such a head is shown in German Pat. No. 1,801,338 to Bonny. Two Hall elements are connected in a differential fashion so that the output voltage of the head is a function of the difference between the magnetic fields sensed separately by the two units. By incorporating such a differential connection, the effect of stray magnetic fields on the read head is minimized. As is the case with single unit prior art Hall effect read heads, the Hall elements are oriented so that they detect the component of the magnetic field which is perpendicular to the surface of the recording medium. Because of this configuration, heads of this type have limited usefulness in reading high density digitally recorded information.